1. Field of the Invention
The field of the present invention is gas turbine engines and, more particularly, to apparatus and methods for operating gas turbine engines having premixers for providing premixed fuel and air for combustion therein.
2. Description of the Prior Art
Gas turbine engines using external premixers are known, including single stage combustion systems with convection cooled combustors, based on the previous work of the present inventor, such as are disclosed, e.g., in U.S. Pat. No. 5,377,483; U.S. Pat. No. 5,477,671; U.S. Pat. No. 5,481,866; U.S. Pat. No. 5,572,862; U.S. Pat. No. 5,613,357; U.S. Pat. No. 5,638,674; and U.S. Pat. No. 6,220,034. These systems provide close control of the fuel/air ratio by premixing all of the fuel for combustion with essentially all the combustion air using a premixer having a venturi-type mixing tube, and introducing the mixture to the combustion zone of the combustor. Significant reductions in gaseous and particulate emissions have been achieved over a broad range of operating conditions by gas turbine engines, employing the inventions disclosed in the above-listed patents.
However, single stage combustors with external premixers can experience “flashbacks” from the combustor into the premixer, which can occur when the flame speed is greater than the velocity of the fuel/air mixture in the premixer exit. Flashbacks can adversely affect the mechanical integrity and performance of the premixer system and related structure. Flashbacks occur generally during low power operation at the time of minimum premixer exit velocities. Also, pressure “pulsations” in the combustor can occur when the pressure drop across the premixer exit is low, again typically during minimum velocity conditions, due to a feedback effect on the fuel/air mixture flow rate from the premixer.
Moreover, simply decreasing the exit area to increase velocity often is not an option, particularly in fixed exit area constructions because it is also desirable to fix the premixer exit area to provide exit velocities less than that which would cause unwanted impingement at high power high exit velocity operation. Impingement of the flow onto the combustor wall will increase heat transfer and increase thermal fatigue. High premixer exit velocities also can reduce combustor volumetric efficiency by limiting combustion until mixture velocities have subsided.
Also, recent tests have shown that operation of engines of the type described in my aforementioned patents can experience structural damage of the combustor, typically during rapid decreases from high power operation. It is believed that the overpressures resulting from such “load shedding,” where a very large portion (≈80%) of the compressed air is forced to travel through the convection cooling channels, can sometimes structurally deform or otherwise damage the combustor which, when heated, has diminished strength.
Apparatus and methods which can diminish the occurrence or severity of one or more of the flashback, pulsations, impingement, and overpressure phenomena in gas turbine engines with external premixers, particularly single stage combustor systems with convective cooling, would be highly useful improvements.